1. Field of Invention
The invention relates generally to methods for synthesizing spiro quarternary ammonium systems. In particular, the invention relates to methods for preparing electrolytes comprising spiro quaternary ammonium salts and to electrolytes formed thereby.
2. Description of Related Art
Spiro quaternary ammonium systems have been found useful in a wide variety of applications. For example, spiro quaternary ammonium salts such as:
have been described by Blicke and Hotelling as novel cholinergic agents for the control of blood pressure (Blicke, F. F.; Hotelling, E. B. J. Am. Chem. Soc. 1954, 76, 5099) and characterized by 2D 1H-13C NMR. (R. A. Aitkin, ARKIVOC 2002 (iii) 63-70).
More recently, and of particular interest to the present invention, spiro quaternary ammonium salts have been described in the preparation of non-aqueous electrolyte compositions, particular for use in energy storage devices such as electrochemical cells and capacitors with high capacity (i.e. ultra-or super-capacitors). (Ue, M., et al., S. J. Electrochem. Soc. 1994, 141, 2989-2996. See also U.S. Pat. No. 6,469,888 (Otsuki) and WO 2005/022571 (Ono).) Electrolytes containing spiro quaternary ammonium salts, especially spiro quaternary ammonium tetrafluoroborate and spiro quaternary ammonium bis(trifluoromethylsulfonyl)imide salts, are chemically and electrochemically stable, are compatible with carbon electrodes, have low viscosity and density, are highly conductive over a broad temperature range, and can provide high energy densities over a wide range of usable temperatures. These and other properties make electrolytes containing spiro quaternary ammonium tetrafluoroborate or bis(trifluoromethylsulfonyl)imide salts ideally suited for use in certain electrochemical storage devices, such as electrical double layer capacitors.
However, conventional methods for producing these salts for electrolyte applications are complicated and uneconomical. For example, one-conventional method of producing a spiro quaternary ammonium tetrafluoroborate electrolyte involves a multi-step process wherein a spiro ammonium halide is first formed via alkylation of either ammonia (v. Braun, et al., Chem. Ber., 1924, 57, 187) or cyclic amines, e.g. pyrrolidine, (v. Braun, et al., Chem. Ber. 1916, 49, 970) with dihalo-alkanes, preferably bromides or iodides. These halides can also be obtained via an organometallic reaction (Thomas, et al., J. Am. Chem. Soc., 2003, EN 125, 29, 8870-88). The halide intermediate is further treated by membrane dialysis in water to form a spiro ammonium hydroxide solution, which can then be reacted with hydrofluoroboric acid to form the desired spiro quaternary ammonium tetrafluoroborate. To be useful in electrochemical devices, it is necessary to dry the spiro quaternary ammonium tetrafluoroborate composition to lower its water content. Finally, the spiro ammonium tetrafluoroborate is dissolved in a dried solvent or solvent mixture, and optionally passed through molecular drying sieves, to produce an anhydrous product. Depending on the concentration of water in the product and the desired purity, it may be necessary to employ more than one purification and/or drying step.
Applicants have recognized the need for a more cost effective means of producing electrolytes comprising spiro quaternary ammonium tetrafluoroborate salts. The present invention satisfies these and other needs.